The use of large bags of woven fabric, called bulk bags, to transport bulk quantities of powdered or granular materials, bags which are lifted and moved by forklift trucks and other material handling equipment having hooks or tines, has become commonplace. The cloth for the bulk bags is usually woven of strong, tape-like plastic fibers (e.g., polyethylene), though natural fibers could be employed. Bulk bags of this general type are disclosed in Nattrass et al. U.S. Pat. Nos. 3,961,655 and 4,010,784, in Nattrass U.S. Pat. No. 4,307,764, and in Sandeman et al. U.S. Pat. No. 4,207,937. Persistent problems in the use of these bulk bags arise in discharge of the bulk material from the bulk bag when it has reached its destination. To avoid the necessity of turning the bulk bags upside-down to discharge their contents, the bags have sometimes been provided with bottom discharge closures.
One discharge closure is shown in FIGS. 13-18 of the aforementioned Nattrass et al. U.S. Pat. No. 3,961,655. That particular closure has the disadvantage that the closure projects outwardly of the bottom wall of the bag and may be damaged by careless handling of the bulk bag during use, with resultant loss of part or all of its contents. A relatively flat closure for a bottom discharge opening in a bulk bag is shown in FIGS. 1-12 of the same patent, but that bottom closure relies upon a loose flap that may be displaced when the bag is filled. Furthermore, that closure arrangement also has loose ties projecting outwardly of the bottom wall of the bulk bag, so that the closure could be accidentally opened at the wrong time.
A better bottom closure for a bulk bag is described and claimed in Nattrass U.S. Pat. No. 4,364,424, reissued Dec. 16, 1986 as Reissue U.S. Pat. No. Re. 32,308. The closures shown in that patent afford a relatively flat, reusable closure that protects the contents of the bulk bag through many uses. Another bulk bag closure, is not flat when closed, is shown in Manerba U.S. Pat. No. 4,221,250.
For many applications the bulk bag closures of Nattrass et al. U.S. Pat. No. Re. 32,308 are the best available. There is a definite limitation, however, on the size of the bottom opening in the bulk bag, a limitation that also restricts the rate at which the bulk bag can be emptied. As the outlet opening in a bulk bag bottom wall is enlarged, that wall is weakened. The closure itself makes up in part, but only in part, for this loss of strength. For virtually any bulk bag there is a bottom outlet size large enough so that the bulk bag will no longer pass the 5:1 strength test generally recognized as an industry standard and insisted upon by almost any sophisticated purchaser/user of bulk bags. That size is usually a bottom outlet having a maximum diameter of ten inches (25 cm) or more.